Exploring the temporal mechanism involved in the pitch of unresolved harmonics.

This paper continues a line of research initiated by Kaernbach and Demany [J. Acoust. Soc. Am. 104, 2298-2306 (1998)], who employed filtered click sequences to explore the temporal mechanism involved in the pitch of unresolved harmonics. In a first experiment, the just noticeable difference (jnd) for the fundamental frequency (F0) of high-pass filtered and low-pass masked click trains was measured, with F0 (100 to 250 Hz) and the cut frequency (0.5 to 6 kHz) being varied orthogonally. The data confirm the result of Houtsma and Smurzynski [J. Acoust. Soc. Am. 87, 304-310 (1990)] that a pitch mechanism working on the temporal structure of the signal is responsible for analyzing frequencies higher than ten times the fundamental. Using high-pass filtered click trains, however, the jnd for the temporal analysis is at 1.2% as compared to 2%-3% found in studies using band-pass filtered stimuli. Two further experiments provide evidence that the pitch of this stimulus can convey musical information. A fourth experiment replicates the finding of Kaernbach and Demany on first- and second-order regularities with a cut frequency of 2 kHz and extends the paradigm to binaural aperiodic click sequences. The result suggests that listeners can detect first-order temporal regularities in monaural click streams as well as in binaurally fused click streams.

Adaptive threshold estimation with unforced-choice tasks.

This paper evaluates an adaptive staircase procedure for threshold estimation that is suitable for unforced-choice tasks-ones with the additional response alternative don't know. Within the framework of a theory of indecision, evidence is developed that fluctuations of the response criterion are much less detrimental to unforced-choice tasks than to yes/no tasks. An adaptive staircase procedure for unforced-choice tasks is presented. Computer simulations show a slight gain in efficiency if don't know responses are allowed, even if response criteria vary. A behavioral comparison with forced-choice and yes/no procedures shows that the new procedure outdoes the other two with respect to reliability. This is especially true for naive participants. For well-trained participants it is also slightly more efficient than the forced-choice procedure, and it produces a smaller systematic error than the yes/no procedure. Moreover, informal observations suggest that participants are more comfortable with unforced tasks than with forced ones.

Slope bias of psychometric functions derived from adaptive data.

Several investigators have fit psychometric functions to data from adaptive procedures for threshold estimation. Although the threshold estimates are in general quite correct, one encounters a slope bias that has not been explained up to now. The present paper demonstrates slope bias for parametric and nonparametric maximum-likelihood fits and for Spearman-Kärber analysis of adaptive data. The examples include staircase and stochastic approximation procedures. The paper then presents an explanation of slope bias based on serial data dependency in adaptive procedures. Data dependency is first illustrated with simple two-trial examples and then extended to realistic adaptive procedures. Finally, the paper presents an adaptive staircase procedure designed to measure threshold and slope directly. In contrast to classical adaptive threshold-only procedures, this procedure varies both a threshold and a spread parameter in response to double trials.

Alternative perceptual states 'apparent motion' and 'perceived simultaneity' lead to differences of induced EEG rhythms.

An investigation of the cortical response (EEG) to periodically presented stimuli producing an ambiguity between long-range apparent motion and flicker is reported. ERPs to stimulus onsets differed slightly between the two percepts, in accordance with the results of Manning et al. (1988), Selmes et al. (1997). Induced rhythms exhibited a strong increase in induced beta and gamma powers at electrode positions T7 and T8 during the perception of apparent motion in two out of 10 participants. In addition, a small overall increase in alpha power at 12-13 Hz and a decrease in delta power below 3.5 Hz during perceived motion were found. The results indicate that a variety of different neural rhythms are involved in the perception of long-range apparent motion.

Effects of consciousness on human brain waves following binocular rivalry.

When the two eyes of an observer are exposed to conflicting stimuli, they enter into binocular rivalry and the two possible percepts will alternate in dominance. We investigated neural activity and its time course following binocular rivalry by measuring human event-related brain potentials to transitions from rivalrous to non-rivalrous stimulation. When these changes did not entail a change in conscious perception they elicited a markedly attenuated N1 component and a delayed and attenuated P3 peak as compared to percept-incompatible changes and non-rivalrous control conditions. These results suggest that in humans binocular rivalry is resolved at latest in extrastriate visual areas.

Human event-related brain potentials to auditory periodic noise stimuli.

Periodic noise is perceived as different from ordinary non-repeating noise due to the involvement of echoic memory. Since this stimulus does not contain simple physical cues (such as onsets or spectral shape) that might obscure sensory memory interpretations, it is a valuable tool to study sensory memory functions. We demonstrated for the first time that the processing of periodic noise can be tapped by event-related brain potentials (ERPs). Human subjects received repeating segments of noise embedded in non-repeating noise. They were instructed to detect the periodicity inherent to the stimulation. We observed a central negativity time-locked on the periodic segment that correlated to the subjects behavioral performance in periodicity detection. It is argued that the ERP result indicates an enhancement of sensory-specific processing.

Psychophysical evidence against the autocorrelation theory of auditory temporal processing.

Nowadays, it is widely believed that the temporal structure of the auditory nerve fibers' response to sound stimuli plays an important role in auditory perception. An influential hypothesis is that information is extracted from this temporal structure by neural operations akin to an autocorrelation algorithm. The goal of the present work was to test this hypothesis. The stimuli consisted of sequences of unipolar clicks that were high-pass filtered and mixed with low-pass noise so as to exclude spectral cues. In experiment 1, "interfering" clicks were inserted in an otherwise periodic (isochronous) click sequence. Each click belonging to the periodic sequence was followed, after a random portion of the period, by one interfering click. This disrupted the detection of temporal regularity, even when the interfering clicks were 5 dB less intense than the periodic clicks. Experiments 2-4 used click sequences that showed a single peak in their autocorrelation functions. For some sequences, this peak originated from "first-order" temporal regularities, that is from the temporal relations between consecutive clicks. For other sequences, the peak originated instead from "second-order" regularities, relative to nonconsecutive clicks. The detection of second-order regularities appeared to be much more difficult than the detection of comparable first-order regularities. Overall, these results do not tally with the current autocorrelation models of temporal processing. They suggest that the extraction of temporal information from a group of closely spaced spectral components makes no use of time intervals between nonconsecutive peaks of the amplitude envelope.

Temporal and spectral basis of the features perceived in repeated noise.

When a half-second segment of a noise is played repeatedly, it initially creates a "whooshing" perception. With longer listening, however, individual features like "clanks" and "rasping" emerge. It is easy to tap the period of the perceived structure. This offers a possibility to investigate the mechanisms underlying the perception of these distinct features. The present study addresses the subject of the temporal and spectral extent of the physical correlates of these percepts. Five subjects participated in this study, and their tapping is in notable, although not perfect correlation. The physical correlate of the features can be confined in time to intervals as small as 100 ms. This segment of the stimulus is processed largely independently from the rest of the noise sample. Spectral processing is, in general, local. Some features, however, are spread over more than one octave. In 3 cases out of 25, across-channel processing is apparent.

On the consistency of tapping to repeated noise.

Repeated noise at 1-4 cycles per second evokes an effortless heard rhythmic sensation which is often heard as "clanks" and "rasping." Tapping in synchrony with the period of the perceived structure is easy and consistent within one presentation. The present study addresses the question of whether the tapping to presentations at different times is consistent across presentations and across subjects. Nine listeners from three countries were presented with repeated Gaussian noise samples in 300 separate cyclical presentations. Nine samples of Gaussian noise with sample lengths ranging from 500 to 700 ms were used. In each of the presentations, one of these samples was selected at random and presented cyclically with transientless juxtapositions. The listeners were instructed to tap in synchrony with the perceived structure (i.e., once per period). Tapping to later presentations of a given sample was found to be consistent with prior tapping to the same sample: In most cases, one or two different tapping points per noise sample could be reproduced in different presentations. In the case of two possible tapping points in different presentations, the two points are usually far away from each other (most likely half a period away). The correlation between subjects is noticeable, although not perfect. The correlation between subjects of the same country is not significantly higher. The noise generating algorithm is given explicitly to allow subsequent studies to use exactly the same noises.

Poisson signal-detection theory: link between threshold models and the Gaussian assumption.

The Gaussian model of signal detection cannot fit asymmetric data as long as the variances of the distributions are kept equal. It is therefore common practice to assume unequal variances in order to fit these data. But this assumption leads to the well-known crossover problem. The present paper provides new arguments for the abandonment of the Gaussian model with unequal variances. In its stead, this paper reevaluates multiple-parallel-threshold models. In particular, the Poisson model turns out to be very useful: it can handle data with any degree of asymmetry, giving a reasonable interpretation of the two parameters of the receiver-operating characteristic. The three-state-threshold model (Krantz, 1969) is given a new interpretation in light of the Poisson model. The slope of Poisson double-probability plots turns out to be much closer to unity than is predicted by the Gaussian approximation.

Simple adaptive testing with the weighted up-down method.

This paper proposes a method for adaptive testing that is less complicated than the commonly used transformed up-down methods (1 up 2 down, 1 up 3 down, etc.). In addition, the weighted up-down method can converge to any desired point of the psychometric function. The rule is very simple: Each correct response leads to a decrease in signal level, each incorrect response to an increase. The only difference from the simple up-down method (1 up 1 down) is that the steps upward and the steps downward are of a different size. The straightforward construction of the novel procedure pays off in efficiency and stability: A Monte Carlo simulation reveals a definite advantage, though small, of the weighted up-down method over the 1-up-2-down rule.

A single-interval adjustment-matrix (SIAM) procedure for unbiased adaptive testing.

A new unbiased adaptive procedure is described that requires only half as many presentations in achieving the same precision as the well-known two-interval forced-choice (2IFC) 2-step procedure. The procedure is based on a yes-no task which avoids redundant presentation time. Furthermore, certain psychophysical studies can only be realized with yes-no tasks. Every trial contains randomly presented signals or noises and the answer is either yes or no. The outcome (hit, miss, false alarm, correct rejection) is taken into account by adjusting the signal level in a staircase manner. The adjustment matrix is set up to induce a neutral response criterion. Its convergence point can be adjusted at will. The single-interval adjustment-matrix (SIAM) procedure is compared to von Békésy and 2IFC transformed up-down methods using a Monte-Carlo simulation. The SIAM procedure proves to be the fastest of the unbiased procedures. A test on four subjects verified these results. Implications for optimum track length and the number of reversals to discard are discussed.

On Riccati equations describing impedance relations for forward and backward excitation in the one-dimensional cochlea model.

Recent experimental observations of otoacoustic emissions suggest the existence of spontaneous emitters of sound on the basilar membrane. These tend to send off waves not only in the normal direction of propagation. It is therefore significant to study the environmental conditions such an emitter finds inside the cochlea. The impedance relations seen by these emitters are described by the Riccati equation for an inhomogeneous transmission line. The results reported in this paper differ considerably for forward and backward excitation. This reflects the quite different behavior of the cochlea pertaining to waves traveling forward and backward. Because of reflections, backward waves cannot be treated with the Liouville-Green approximation.

Forward and reverse waves in the one-dimensional model of the cochlea.

Consideration of a source of oto-acoustic emission in a cochlear model implies consideration of the types of waves that such a source can emit. One wave travels in the normal, forward, direction. As any other forward wave it undergoes little or no reflection and it eventually disappears completely because of dissipation. The other wave travels in the reverse direction and it appears to undergo appreciable reflection. In the present paper this phenomenon is studied via the use of two appropriately simplified long-wave models of the cochlea. One model, the exponential model, puts emphasis on the variation of the stiffness along the length of the basilar membrane. The second model concentrates on what happens in the region of resonance. The latter model turns out to have the largest predictive power for the problem at hand. Consideration of the flow of energy in the cochlear fluid brings forth the explanation why in the used model of the cochlea reflection conditions at the stapes have such a surprisingly small influence on the operating conditions of a potential source of emission.